The healthcare reform law may be specific about what health insurers have to cover starting in 2014, but there’s a group that’s left out – people already covered by Medicaid in many states, researchers reported on Monday.

Whether your Medicaid coverage will pay for basic preventive services such as colon cancer screening or vaccines depends on what state you live in, the team at George Washington University found.

The result is a patchwork of coverage that could create an underclass of people who don’t get basic, recommended health care, health policy expert Sara Wilensky and colleague Elizabeth Gray report in the journal Health Affairs.

And many states are not at all clear about what they’ll pay for – meaning doctors take a gamble if they do provide the services. Many doctors may end up not recommending care if there’s doubt about whether they will get paid for it, said Wilensky.

“When providers are unsure about what covered or what is not covered, there is a good chance that you won’t be getting those services,” Wilensky told NBC News.

“It’s really hard to tell what states cover,” Wilensky said in a telephone interview. She and Gray spent six months calling Medicaid officials in each state to try to sort out how they decided what to pay for.

“It shouldn’t be that hard. The easiest thing for states to do is to say we cover all U.S. Preventive Services Task Force A and B rated services,” Wilensky said.

But many don’t. The U.S. Preventive Services Task Force (USPSTF) is an independent panel of experts that does regular reviews of the scientific evidence on preventive health care. It’s the group that recommended that women can usually wait until the age of 50 to start getting regular mammograms, and that said men don’t need a regular blood test to screen for prostate cancer.

If it gives a recommendation of an A or B grade, that means it’s highly certain that a service is either substantially or at least moderately beneficial.

“Federal policy makers highlighted the importance of covering these preventive services by requiring most insurers to do so, yet coverage for existing adult Medicaid beneficiaries is left to state discretion,” Wilensky and Gray wrote.

Under the 2010 healthcare reform law known widely as Obamacare, insurers have to pay for anything rated A or B by the USPSTF if they want to offer policies on the new health insurance exchanges. In states that choose to offer Medicaid to more people as part of healthcare reform, Medicaid also has to pay for these services for the newly covered patients. And Medicare, the federal health insurance plan for the elderly, pays for them.

The argument is that these tests, screening tests and health services work. Screening for cancer helps catch it before it gets serious and expensive to treat. Vaccines prevent diseases that put people into the hospital. Screening for heart disease symptoms saves money and prevents illness, said Nancy Brown, CEO of the American Heart Association, which helped pay for the study.

“By lowering risk factors such as high blood pressure and cholesterol, Americans can reduce their risk of heart disease or stroke by as much as 80 percent,” Brown said in a statement. “Evidence-based screenings play an essential role in identifying and reducing these factors.”

Despite these compelling arguments for coverage, states are surprisingly hit and miss in what they do require, Wilensky said. For instance, California has two different standards for offering mammograms to Medicaid patients – one of its Medicaid plans pays starting at age 40, while another pays starting at age 50.

“Colorectal cancer screens are explicitly covered by 33 states, and another 10 states are likely to cover this service through an age-appropriate screen,” the researchers wrote. But that leaves seven states with unclear policies.

“There is substantial variation among states in terms of coverage for sexually transmitted diseases (STD) and HIV screening, and there are often restrictions on coverage,” they added.

Maine was the only state that had clear rules for paying for all USPSTF recommendations, Wilensky said. Texas covered most – although it covers very few patients on its Medicaid program and will not be expanding Medicaid to more people next year. Oregon, Delaware and Colorado covered many of the recommended services.

But others were a mixed bag. “I certainly think states could do a lot better letting beneficiaries know what coverage they are offering,” Wilensky says.

She said Congress could make it easy for everyone by requiring the entire Medicaid program to cover these services, instead of leaving it up to states.

“If these services are so crucial to good health that coverage of them is required for so many others, why were people who are currently on Medicaid left off of health reform’s prevention bandwagon?” she wrote.

A second study in Health Affairs might help answer whether the confusion keeps doctors from even treating Medicaid patients in the first place. Sandra Decker of the National Canter for Health Statistics found that 30 percent of doctors nationwide turn away new Medicaid patients.

"Nearly one-third of office-based physicians did not accept new Medicaid patients in 2011–12—a figure that was higher for primary care physicians than for others," she wrote. "Primary care physicians in New Jersey, California, Alabama, and Missouri were the least likely to accept new Medicaid patients in 2011–12."

This ranged from 8.9 percent of doctors in Minnesota to 54 percent in New Jersey.

Decker didn't speak to the doctors so she did not ask why they turned them away, but said the data can act as a baseline to see if the healthcare reform law encourages more doctors to take on Medicaid patients.